Nearly one in five Americans now incorrectly believes that President Obama is a Muslim, evidence of growing confusion among the population about the president's faith.

The new poll from the nonpartisan Pew Forum on Religion and Public Life found that 18 percent of those surveyed wrongly identified Obama as Muslim, up from 11 percent in March 2009. At the same time, the number of Americans who knew correctly that Obama is Christian has declined from 48 percent in March 2009 to 34 percent today. But 43 percent of Americans now say they don't know what Obama's religion is at all.

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Watch "World News with Diane Sawyer" for more on this story tonight on ABC.

The misinformation continues to exist despite the president's own declarations of his Christian faith and the statements of his spiritual advisors.

"The president is, obviously, he's Christian. He prays every day," White House spokesman Bill Burton said today aboard Air Force One.

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"He communicates with his religious advisor every single day," Burton said. "There's a group of pastors that he takes counsel from on a regular basis. His faith is very important to him, but it's not something that's a topic of conversation every single day."

Burton said the president has talked "extensively" about his faith in the past and "you can bet he'll talk about his faith again." But "making sure Americans know what a devout Christian he is" is not the president's top priority.

The poll indicates that groups who have shown the most willingness to believe the wrong notion that the president is a Muslim include conservative Republicans, 34 percent of whom believe Obama is Muslim. Eighteen percent of independents say the president is a Muslim, up from 10 percent in March 2009.

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"I think the reality is that false beliefs spread like gossip more than actual information," said Andrew Perrin, an associate professor of sociology at the University of North Carolina, Chapel Hill.

Perrin's research has shown that a false perception can spread quickly if people's friends and neighbors also have heard or believe a similar idea.

"False beliefs propagate when people think others believe them and when they have a supportive source that wants them to hold it," Perrin said.

Perrin has found that even direct denials of the false information do not always solve the problem.

"In my own research, when [people] get reliable information that discounts these beliefs, they tend to cling to those beliefs more," Perrin said.

Misperception Also Exists Among President's Political Allies

But even among the president's allies, the numbers are shifting. In March 2009, 55 percent of Democrats said the president is a Christian. That number is now 46 percent.

African-Americans, who voted for Obama overwhelmingly, have shown a similar shift. In March 2009, 36 percent of African-Americans said they didn't know the president's religion. That number is now 46 percent. Self-described liberal Democrats who don't know what religion the president is shifted from 23 percent to 31 percent.

President Favors a Private Faith

Obama has favored a more private faith since he took office in January 2009, attending formal church services just a handful of times, including Easter of this year.

Like his predecessor George W. Bush, President Obama has said he prefers worshiping at the chapel at Camp David. But Obama rarely goes to the presidential retreat, instead spending Sundays at the White House. Weekend activities often include basketball or golf.

"My faith and neighborhood initiatives director, Joshua DuBois, he has a devotional that he sends to me on my BlackBerry every day," Obama said. "That's how I start my morning. You know, he's got a passage, scripture, in some cases, quotes from other faiths to reflect on."

Obama: Faith Has Deepened While in Office

The president said during the same interview that his faith had deepened in his time in office.

"[Before taking office], I had a habit of praying every night before I go to bed. I pray all the time now," Obama laughed. "Because I've got a lot of stuff on my plate, and I need guidance all the time."

While the president and his family initially planned to join a church in Washington, they put the search on hold after finding the trappings of the modern presidency too cumbersome for congregation life. Before the president attends a service, the building must be swept for threats and every churchgoer screened for weapons.

An Obama family visit to the 19th Street Baptist Church, a historic African-American congregation in Washington, turned into a circus atmosphere that dismayed the family, according to aides, particularly after learning that longtime church members were turned away from the service.